Mazda recently introduced a new kinetic-energy recovery system it calls i-ELOOP—a play on “intelligent energy loop.” i-ELOOP is very similar to BMW’s EfficientDynamics setup in that neither system sends recovered energy back into the powertrain for a small power burst, as do hybrid systems.

What makes Mazda’s version different from BMW’s is how it stores energy. When a car equipped with i-ELOOP is decelerating, a variable-voltage alternator (12 to 25 volts) pumps electricity into an electric double-layer capacitor (also known as a supercapacitor). When the car comes to a stop, Mazda’s engine stop-start system—branded i-stop—takes over and shuts the engine off. At this point, all auxiliary vehicle functions (radio, HVAC, headlights, etc.,) are powered by the supercapacitor; its 25-volt output is stepped down to 12 volts by a DC/DC converter. There are times that the supercapacitor will recharge the 12-volt battery, too. Rather than a supercapacitor, BMW’s EfficientDynamics uses an expensive glass-mat battery; the glass-mat battery is used because it can be reliably discharged to a lower state of charge than regular lead-acid lumps.

If there’s an argument against the i-ELOOP approach, it’s that supercapacitors self-discharge fairly rapidly and only store electricity temporarily, meaning that they must be topped off before each stop. But this isn’t actually much of a problem, as it takes just a few seconds to fully charge the capacitor. And, unlike batteries, supercapacitors have an extremely long life (millions of cycles) and likely will never have to be replaced.

It sounds a bit complicated, but the payoff, we suspect, is that the air conditioning and other auxiliary devices won’t force the engine to run at idle as often, ultimately saving a few gallons of gasoline. For its part, Mazda claims i-ELOOP is good for a 10-percent fuel-economy improvement in stop-and-go traffic.

As for i-ELOOP’s U.S. prospects, our understanding is that the system is likely to be offered here, but not for some time.